The United States says it has carried out an "extraordinary" flight
over Ukraine under an international military surveillance treaty,
amid what it called a pattern of “increasingly provocative and
threatening activity" by Russia.

The Pentagon said the flight took place on December 6, with tensions
soaring between Ukraine and Russia following a naval confrontation
last month.

CNN has reported that the United States is also making plans to sail
a warship into the Black Sea, but that has not been confirmed.

Ukrainian government forces have been fighting against Russia-backed
separatists in eastern Ukraine since April 2014, shortly after
Russia military forces seized Ukraine's Crimean Peninsula and
illegally annexed the territory.

Tensions between Kyiv and Moscow escalated on November 25, when
Russia fired on Ukrainian naval vessels that were attempting to pass
under a massive bridge that spans the Kerch Strait and links Russia
with Crimea.

Russia ultimately seized three Ukrainian ships and 24 crewmen, who
remain in Russian captivity despite international calls to free
them.

“Today, the United States and allies conducted an extraordinary
flight” under the Open Skies Treaty to “reaffirm U.S. commitment to
Ukraine and other partner nations,” the Defense Department said in a
statement.

Since 2002, the Open Skies Treaty has allowed 34 signatory states to
send unarmed observation flights over one another's territory.

These flights are usually scheduled well in advance, but the treaty
allows "extraordinary," or unscheduled flights, if two participating
members agree -- in this case Ukraine and the United States.

Russia's “unprovoked attack” on Ukrainian vessels in the Black Sea
is “a dangerous escalation in a pattern of increasingly provocative
and threatening activity,” the Pentagon also said.

It added that the United States “seeks a better relationship with
Russia, but this cannot happen while its unlawful and destabilizing
actions continue in Ukraine and elsewhere.”

U.S. surveillance planes and drones regularly skirt Russia's Black
Sea coastline, as well as that of Crimea. Russian jets have been
shown regularly confronting, and shadowing, the surveillance planes.

CNN reported on December 5 that U.S. military officials have asked
the State Department to notify Turkey of possible plans for a Navy
ship to pass through the Bosphorus and the Dardanelles, the waterway
that connects the Mediterranean to the Black Sea.

The channel cited three unnamed U.S. officials as the source for the
report, which could not be immediately confirmed.

A decades-old treaty requires that all countries that do not have a
Black Sea coastline give Turkey 15-days notice when their warships
plan to transit the waterway.

Two of the unnamed officials told CNN that the plans weren't set in
stone, and that the notification was merely to provide the Navy with
the option to move a warship into the area.

"We routinely conduct operations to advance security and stability
throughout the U.S. 6th Fleet area of operations to include the
international waters and airspace of the Black Sea," said Commander
Kyle Raines, a spokesman for the U.S. Sixth Fleet, which oversees
naval operations in the region.

"We reserve the right to operate freely in accordance with
international laws and norms," he added.

Konstantin Kosachev, who heads the international affairs committee
in Russia’s upper house of parliament, the Federation Council, said
the U.S. military presence in the Black Sea will only increase
tension in the region, according to the TASS news agency.

Viktor Bondarev, the chairman of the Federation Council’s committee
on defense and security, called the possible deployment of U.S. Navy
ships in the Black Sea as “yet another episode of saber rattling,
yet another ploy to humiliate Russia," TASS reported.

NATO,
which includes several Black Sea member states, and the United
States have routinely sent warships patrolling the Black Sea,
despite Russian concerns.

After the five-day Russia-Georgia war in 2008, with Russian forces
still encamped on shore nearby, Washington anchored the flagship for
the U.S. Sixth Fleet -- the USS Mount Whitney -- off Georgia's coast
in a sign of support for the country.

Under a 2003 treaty, Russia and Ukraine agreed to share access to
the Sea of Azov. However, since the 2014 annexation, and the
completion of the Kerch bridge earlier this year, Russia has slowly
restricted access for Ukrainian ships.

According to CNN, the last U.S. ship to enter the Black Sea was the
fast transport ship USNS Carson City in October.